This invention relates to a weighing apparatus and particularly, but not exclusively, to a portable weighing apparatus for weighing vehicles.
In our copending United Kingdom Patent Application No. 30592/78 (Publication No. 2001766A) we have described and claimed a weighing apparatus which comprises a resilient body formed of an elastomeric material, a closed cavity provided in said body, a fluid contained in said cavity, conduit means connecting the cavity to indicator means for indicating changes in volume of the cavity when the body suffers changes due to compression forces applied thereto by the member being weighed, said body having a resilient metal plate or bars fixed to an upper surface thereof, a resilient metal load support plate mounted on said first mentioned plate or bars and a further resilient metal plate embedded in the body and spaced below the first mentioned plate or bars and to which reaction forces are applied during weighing.
The properties of an elastomeric material, such as natural or synthetic rubber, change due to the effects of temperature, the composition of the elastomeric material and age. For example at low temperature the material stiffens and becomes less flexible and at high temperature the material softens and becomes more flexible. It was therefore thought necessary to provide a read-out scale for the weighing device which was calibrated to cater for the properties of the elastomeric material at a particular ambient temperature and this involved having to provide a range of differently calibrated scales from which done could be chosen for the ambient conditions in the area of use of the weighing apparatus. For example a different scale would be provided where the apparatus is in hot climates compared with the scale used in cold climates. Also, there may be composition changes between batches of elastomeric material and therefore a calibrated scale for a weighing apparatus formed from one batch could not necessarily be used on a weighing apparatus formed from another batch.
We have found that if the deformity of the wall of the cavity can be changed then a common calibrated read-out scale can be used under a wide range of ambient temperature conditions, and for weighing apparatus formed of different compositions of elastomeric material, and thus obviate the need to have a plurality of differently calibrated scales available.